In a pneumatic tire, in order to exert excellent driving stability by improving casing stiffness, what is normally performed is to bury, inside a sidewall portion, a reinforcement rubber extending from a bead portion to the sidewall portion. However, if the sidewall portion is made thinner for the purpose of making a tire lighter, the reinforcement rubber layer has to be inevitably made thinner. As a result, that leads to a reduction in stiffness of the tire. To complement the reduction in stiffness of the type, it is required that the reinforcement rubber layer be arranged to reach the highest possible position. A variety of related arts for that purpose have been proposed (refer to Patent Documents 1 to 5, for example).    Patent Document 1: Japanese patent application Kokai publication No. Hei8-318713    Patent Document 2: Japanese patent application Kokai publication No. Hei8-318714    Patent Document 3: Japanese patent application Kokai publication No. Hei9-300923    Patent Document 4: Japanese patent application Kokai publication No. Hei11-28916    Patent Document 5: Japanese patent application Kokai publication No. 2001-71715
However, since a large amount of deformation occurs in a portion from a widest-width position of the tire to a shoulder portion of the tire when the tire is run, if rubber having a high degree of hardness, which has been conventionally used, is used as it is, there are such problems as: reduced durability of the tire, because of a small breaking elongation; and deteriorated rolling resistance, because of a large hysteresis loss.